The present invention is related to computer display systems and, more particularly, to liquid crystal display systems for portable computers.
Humans can sometimes perceive images which are technically different images to be essentially the same. In the case of brightness intensity, it is known that the human eye has a logarithmic response (brightness must increase exponentially to produce what the eye perceives as linearly increasing intensity). In addition, due to a finite response time, the human eye integrates an image over time (temporal integration). The human eye is also limited in terms of resolution and this cause an image to be integrated spatially (spatial integration).
These factors of human perception must be properly coordinated in order to yield a display system with good visual display quality.
One of the earliest display technologies to employ gray scaling, creation of levels of brightness, is the cathode ray tube (CRT). Gray scaling on a CRT is achieved through the use of analog voltage levels of the input signal. The signal is then converted to a control voltage for the grid electrode which, in turn, controls the electron beam intensity as the beam sweeps across the CRT phosphor. A higher electron beam intensity corresponds to a brighter image on the CRT phosphor. The analog voltage levels can, in theory, be infinitely small and thus, the number of gray scales which can be produced are infinite.
Although the analog method of producing gray scales on the CRT is perhaps the most commonly used method, especially for television, and is generally regarded optimum, other gray scale techniques, spatial and temporal, have also been employed on CRTs to meet cost and various other design goals with various levels of display quality.
A major goal of any display system for producing gray shades should be that the number of gray shades be large (64 to 256), the gray shades be linear, the gray shades be stable (no flicker or jitter), the gray shades be smooth (non-grainy), and the system be cost-efficient.
Over the past years, other types of display technologies have emerged. Many of these technologies are multiplexed displays. These types of displays have typically two electrodes to apply stimulus to the individual display elements (pixels). In order to reduce the number of display connections, the electrodes are arranged in groups of rows and columns. With this arrangement, it is possible to scan these types of displays 1-pixel at a time, similar to a CRT. Given this type of scanning, the drive period for a given pixel is inversely proportional to the total number of display pixels in the display. For many of these multiplexed displays, the drive period, or duty cycle, (time driven/total time) determines how well the display is able to perform in terms of brightness and the contrast ratio (contrast ratio=on-brightness/off-brightness).
A common display used in portable computers has 640 vertical columns and 480 horizontal rows. Since all columns are driven simultaneously, the duty cycle is usually referred to as 1/480. It is also possible for a 480 line display to have a 1/240 duty cycle. These displays are referred to as "dual scan" because the display column electrodes are disconnected in the middle of the display and a separate set of column drivers are required to drive the top and bottom portions of the display. These dual scan displays have become popular because the display characteristics, i.e., the contrast ratio, are better.
With many of the display technologies, a more efficient way to drive these displays is to energize a group of pixel elements at the same time. One common grouping is an entire row of pixels at the same time. This usually means that the data for the individual column electrodes must be gathered and stored such that all columns can be driven simultaneously for the given row period.
The analog gray scale is well suited in terms of cost and display quality for some display types, such as the CRT. However, the analog technique is not possible with some new display technologies because the display elements are simple on/off devices, such as AC plasma displays, and not practical with some others due to cost or design complexities.
Super-Twisted-Nematic (STN) liquid crystal displays (LCDs) have provided the proper sharp threshold voltages needed for high duty cycle displays. Over the past decade these displays have been used extensively in portable, battery-operated computers. These portable computers (laptop, notebook, sub-notebook, palmtop, etc.) have enjoyed increasing popularity. The inherent low power capability of the LCD has enabled these products to achieve light weight and low power.
Another type of LCD display is the active matrix display, which has even better display characteristics than STN displays. Analog gray scales are possible on these types of display panels, but the cost of the display technology and the drive electronics required by these displays have limited these panels to the high-end niche of the portable computer market.
STN LCDs in particular do not lend themselves well to analog drive techniques because the voltage difference between on and off states is very small and, thus, difficult to control. Spatial and temporal techniques are used to control gray scales on these type of displays.
Spatial techniques have been employed for years in the printing industry. This technique is called "halftoning". In this technique "dots" of various sizes are used. Large black dots (on a whitepaper) make the image in that area darker and smaller black dots make the image in those areas lighter. When viewing these images from "normal" reading distances, the dots are not noticeable and the image appears to be made from solid shades. The human visual systems limitation in spatial resolution effectively makes this technique possible.
Another shading technique is "dithering". Raster printers do not have the ability to vary the size of an individual dot, so groups of dots are used. In dark areas of the image a higher proportion of black dots is used, while in the lighter image areas a lower proportion of black dots is used. Even with the relatively high dot density of today's raster printers 300-600 DPI (dots per inch), various dithering techniques have been employed to make these shades appear smoother, i.e., less grainy. Two popular spatial techniques in this field have been ordered dithering and error diffusion.
Early systems which employed an LCD display have applied spatial techniques to produce gray scales. However, even when complex dithering techniques were employed, the pictures still appeared very grainy. This was due to the much lower DPI densities of typical computer displays (50-100 DPI versus 300-600 DPI for printers).
Temporal techniques are also employed in many displays including LCDs. This basic technique controls the proportion of time that a pixel is on and off. This technique takes advantage of the time integration of the human eye. Indeed, many room light dimmer controllers use this technique to control the apparent intensity of a light bulb. In the case of the light bulb dimmer control, there are two factors which keep the human visual system from observing flicker. The first is the integration of the human eye (as discussed previously) and the second is the integrating nature of the bulb itself (bulb intensity changes are relatively slowly compared to the 60 Hz line frequency).
The term CFF (Critical Flicker Frequency) is the frequency below which humans can perceive "flicker". The term, "flicker," is typically defined as any change in display intensity over time. Flicker may be an overall "beating" or "pulsing," or it may be a "jittezing" or "motion" (like a movie marquee). The human visual system is more sensitive to flicker in the middle brightness levels. The response time of the display itself also has a major impact on the CFF. CRTs with slow phosphors have a lower CFF than fast phosphor CRTS.
A common temporal technique is pulse width modulation (PWM) of which the room light dimmer is a good example. The 60 Hz AC drive voltage is delivered to the bulb for a percentage of the drive interval. The lower the percentage, the lower the average bulb intensity.
It is possible to use PWM techniques on CRTs by dividing the pixel time into sub-intervals. However, this is generally not done since the times involved for a single pixel is on the order of 20 to 60 nanoseconds. The PWM technique has been used on multiplexed panels, such as electoluminescent (EL) and plasma panels. The horizontal drive period is divided into sub-intervals (usually binary weighted time intervals). The smallest possible sub-interval may be 1-pixel time. The drive electronics required for these displays is more expensive than for the STN LCDs because the drivers must store multiple bits per pixel, i.e., 16 gray levels require 4-bits per pixel of storage. Since an entire ROW is buffered before the row is displayed, the pixel information must be stored in the column drivers. A 16-level driver (requiring 4-bits per pixel) would require 4 times more memory storage than a 2-level driver. PWM techniques as described do not increase flicker because the modulation takes place during a subinterval of the overall display refresh rate, i.e., 60 Hz.
On the other hand, PWM techniques have generally not been employed successfully on STN LCDs because the fast switching of the high voltage drive electronics causes objectionable "noise," or interference, to be induced on other pixels in the row or column.
One additional PWM technique which does not induce flicker is to drive pixels for a portion of tie frame period (the smallest sub-interval is one row time). This technique has been used successfully on AC plasma panels, but, the cost of the drive electronics is increased even higher above the previously described PWM system. Not only must multiple bits per pixel be stored, but, multiple pixel rows must also be stored. This technique could conceivably be used on STN LCDS, but, the cost most likely limits the success of such a display.
The temporal technique which is widely used with STN LCDs is referred to as Frame Rate Cycling (FRC) and also as Frame Rate Duty Cycle (FRDC). This technique uses the frame refresh period as the smallest time interval. But since the time interval involves multiple frames, flicker is the biggest problem with this technique. The human eye is capable of integrating (smoothing) an intensity, but the basic frequency of the displays intensity variations must be above the CFF point. Present STN LCDs generally have a much lower CFF point than a CRT because the LCD response time is long compared to the CRT. Nonetheless, this only helps a certain amount to reduce flicker. Other factors must be considered in order to reduce flicker.
So far temporal integration techniques which employ time-averaging (PWM, FRC) and spatial integration techniques, such as error diffusion and ordered dithering, have been discussed. Another technique used for years on home TV sets to reduce flicker is interlacing, which takes advantage of both temporal and spatial integration features at the same time. In a television, the frame is broken into two 60 Hz fields. During field-1, the odd scanlines are displayed. During field-2, the even scan lines are displayed. The frame period is 30 Hz, well below the CFF for most CRTs. Since "dips" in intensity in a display region are quickly "filled" by the next field update, the human eye is fooled into thinking that the display is being updated at or near the 60 Hz field frequency. If the eye were able to resolve smaller images, this technique would not be nearly as effective. The interlacing technique was not used to help TVs produce gray scales. Instead, interlacing was used to reduce flicker which occurred when the bandwidth of the TV system was limited to a 15.75 kHz horizontal scan rate.
On the other hand, this flicker reduction technique is the basis of the FRC algorithms used to produce gray scales on STN LCD displays. In order to produce more than two levels of gray shade, more than one frame time is required. Two frame times will yield 3-shades (0, 100%, 50%). The new shade of 50% is the result of the pixel being "on" during FIELD-1 and "off" during FIELD-2. An LCD is not actually interlaced in the even/odd scan lines like the CRT example. Rather, a pixel is DRIVEN to the on state during the first FRAME CYCLE and to the off state in the second FRAME CYCLE. If all pixels of the display are cycling at this rate, then a 30 Hz flicker results. To overcome this, the even lines are driven ON for Field-1, OFF for Field-2 and the odd lines are be driven OFF during Field-1, ON for Field-2. With the lines 180 degrees out of phase with each other, the 30Hz flicker is essentially canceled out (if the eye cannot easily distinguish the 2-lines).
The term, "phase," is often used in FRC algorithm discussions. A 2-frame FRC algorithm is said to have 2-phases to define the temporal sequence. PHASE-1 is "on" during frame-1 and "off" during frame-2, while PHASE-2 is 180 degrees out of phase, or "off" during frame-1 and "on" during frame-2.
In computer graphics horizontal lines are often drawn on the display. If the line has a 50% gray value and the line is only one row thick, then the line flickers at the 30 Hz rate. To avoid this, a technique commonly used in many FRC algorithms is to interlace the rows AND columns. In this method a 2.times.2 matrix of pixels is used. Diagonally adjacent pairs of pixels observe PHASE-1 and the other pair of diagonally adjacent pixels observe PHASE-2. The flicker produced by the one-horizontal-line example described above is avoided since horizontally adjacent pixel pairs are 180 degrees out of phase. Vertical lines (also encountered often in computer graphics) also contain vertically adjacent pixels of opposite phase. Only a checkerboard pattern vulnerable to flicker under this 2.times.2 matrix technique.
Almost all FRC algorithms use a square matrix of pixels in order to avoid the flicker problems described above and to compact the phases into the smallest area possible and thereby take advantage of the spatial resolution limits of the human visual system of course, it is often desired to provide more than 3 gray shades (using 2-frames).
By using 3 frames for the gray scale period, 4 shades may be produced:
0/3=0% PA1 1/3=33% PA1 2/3=66% PA1 3/3=100%
Notice that in this example the 50% shade is missing. If the 50% shade were to be used, then 5 shades are produced. However, the intensity steps between each gray shade are not be equal. However, 5 shades are still better than 4 shades. of course, 5 equally spaced shades produce better looking images than 5 unequally spaced shades. The 3-frame example requires 3 phases for each shade. For example, gray shade 1/3 has the phases spaced 120 degrees apart from each other as shown in FIG. 1. The table in FIG. 2 shows, for instance, that in 8-frame periods it is possible to produce 23 unique shades (adding all the unique shade numbers together).
By combining the basic FRC technique along with the dithering (spatial) techniques, it is possible to extend the number of gray scales to a level beyond those produced by FRC alone even further. It has been demonstrated that a base of 16 gray shades can be extended to 32 or 64 (or beyond) gray shades by using such dithering techniques. However, as mentioned earlier, these images tend to look grainy compared to comparable shades generated by other techniques. It has also been demonstrated that as many as 17 frames have been successfully used to produce 18 shades using the FRC technique with acceptable flicker display quality. It has also been demonstrated that 64 FRC gray shades can be produced with as few as 16 frames with the disadvantage of that the resulting gray shades are not perfectly linear.
Thus an effective technique of providing 64, or more, FRC-generated linear gray shades free of flicker has not been set forth. The present invention solves this problem. Furthermore, the present invention may be easily adapted for color displays.